Cubic boron nitride (cBN) is the material used worldwide for the machining and polishing of ferrous metals due to its extreme hardness, which is second only to diamond. cBN is similar to diamond in both its structure and physical properties such as high strength, thermal conductivity, and atomic density, and it is even more resistant to chemical attack. Despite its promising physical properties, cBN is the least studied wide band gap semiconductor, because the only reliable methods of producing this phase involve high pressures (> 4 GPa) and high temperatures (> 1600 K).

Recently it has been found that the threshold pressure of cBN formation can be drastically reduced when it crystallises from BN solutions in supercritical fluids (ammonia, hydrazine, etc.) [1]. The present work is the first attempt to study in situ regularities of cBN crystallisation in the BN - N2H4 system at high pressures and temperatures employing X-ray diffraction with synchrotron radiation.

The high-pressure experiments were carried out using a large-volume Paris-Edinburgh press. The diffraction measurements were performed in the energy-dispersive mode at beamline ID30.

Our findings have shown that at 4.1 GPa the cooling of the solution containing 81 mol.% BN from 1460 to 1400 K results in a spontaneous crystallisation of cubic boron nitride (Figure 116). The emergence of cBN lines is accompanied by a simultaneous change in the shape of the solution spectrum, in particular by appearance of a broad halo with the maximum at 107 keV (dhkl = 1.1 Å). A similar change in the shape of the solution spectrum was also observed at 4 GPa and 1350 K in cooling the solution containing 33 mol.% BN. In this case, however, no formation of any crystalline phase was recorded even in cooling the solution down to room temperature.

A subsequent heating of the above BN-N2H4 mixtures up to 1600 K in all the cases gave rise to a recovery of the characteristic shape of the solution spectrum due to dissolution of solid phase(s).

The above experimental facts should be attributed to the existence of the L = X + cBN eutectic reaction at 1400±15 K and 4 GPa. This reaction results in the crystallisation of cBN and the formation of an unknown amorphous phase of the BN - N2H4 system (phase X). A hypothetical phase diagram of the BN-N2H4 system at 4 GPa is shown in Figure 117.

Thus, the spontaneous crystallisation of cubic boron nitride was observed for the first time at 4.1 GPa which is the lowest pressure of cBN formation without a catalyst. This result will probably lead to a new method of synthesising high-purity cubic boron nitride at relatively low pressures.

Reference
[1] V.L. Solozhenko, Diamond & Relat. Mater., 4, 1 (1994).

Authors
V.L. Solozhenko (a), Y. Le Godec (b), M. Mezouar (c), J.-M. Besson (b), G. Syfosse (b).

(a) Institute for Superhard Materials, National Academy of Sciences of Ukraine, Kiev (Ukraine)
(b) Physique des Milieux Condensés, Université P&M Curie, Paris (France)
(c) ESRF